Rubber goods such as tire treads often are made from elastomeric compositions that contain one or more reinforcing materials such as, for example, particulate carbon black and silica; see, e.g., The Vanderbilt Rubber Handbook, 13th ed. (1990), pp. 603-04.
Good traction and resistance to abrasion are primary considerations for tire treads; however, motor vehicle fuel efficiency concerns argue for a minimization in their rolling resistance, which correlates with a reduction in hysteresis and heat build-up during operation of the tire. Reduced hysteresis and traction are, to a great extent, competing considerations: treads made from compositions designed to provide good road traction usually exhibit increased rolling resistance and vice versa. Filler(s), polymer(s), and additives typically are chosen so as to provide an acceptable compromise or balance of these properties. Ensuring that reinforcing filler(s) are well dispersed throughout the elastomeric material(s) both enhances processability and acts to improve physical properties. Dispersion of fillers can be improved by increasing their interaction with the elastomer(s).
Certain tests have come to be recognized as correlating certain physical properties of vulcanizates with performance of products, particularly tire treads, made therefrom. For example, higher rebound values and lower loss tangent values (tan δ) at high temperatures have been found to correlate with reductions in heat build-up during operation (hysteresis), higher elastic modulus values at high temperature and strain with better handling performance, and lower modulus values at low temperatures with wet, snow and ice traction. (In the foregoing, “high temperatures” usually are considered to be from ˜50° to ˜65° C. while “low temperatures” from ˜0° to −25° C.)
The section of a polymer chain from the site of the last crosslink to an end of the polymer chain is a major source of hysteretic loss; because a free end is not tied to the macromolecular network, it cannot be involved in an efficient elastic recovery process and, as a result, energy transmitted to this section of the polymer (and vulcanizates in which such a polymer is incorporated) is lost as heat. Ensuring that polymer chain ends are tied to, or otherwise interact well with, reinforcing particulate fillers, is important to many vulcanizate physical properties such as, for example, reduced hysteresis. Chemically modifying the polymer, typically at a terminus thereof, is an effective way of increasing interactivity of fillers and polymers.
Sequential reaction sequences employing siloxanes to provide living polymers with terminal functionality have been described in U.S. Pat. No. 8,063,153 and international patent publ. no. WO 2012/092595. The former also describes that an episulfide compound can be used in place of the cyclic siloxane in the first step of the sequential termination process.